Seismic surveys have become the primary tool of exploration companies in the continental United States, both onshore and offshore. As an example, an onshore seismic survey is conducted by creating a shock wave—a seismic wave—on or near the surface of the ground along a predetermined line, using an energy source. The seismic wave travels into the earth, is reflected by subsurface formations, and returns to the surface where it is recorded by receivers called geophones—similar to microphones. By analyzing the time it takes for the seismic waves to reflect off of subsurface formations and return to the surface, a geophysicist can map subsurface formations and anomalies and predict where oil or gas may be trapped in sufficient quantities for exploration activities.
Until relatively recently, seismic surveys were conducted along a single line on the ground, and their analysis created a two-dimensional picture akin to a slice through the earth beneath that line, showing the subsurface geology along that line. This is referred to as two-dimensional or 2D seismic data.
Currently, almost all oil and gas exploratory wells are preceded by 3D seismic surveys. The basic method of testing is the same as for 2D, but instead of a single line of energy source points and receiver points, the source points and receiver points onshore are commonly laid out in a grid across the property. The resulting recorded reflections received at each receiver point come from all directions, and sophisticated computer programs can analyze this data to create a three-dimensional image of the subsurface.
Conceptually, 3D surveys are acquired by laying out energy source points and receiver points in a grid over the area to be surveyed. The receiver points—to record the reflected vibrations from the source points—are commonly laid down in parallel lines (receiver lines), and the source points are laid out in parallel lines that are typically approximately perpendicular to the receiver lines. Although orthogonal layouts are preferred, non-orthogonal layouts are sometimes used as well. The spacing of the source and receiver points is determined by the design and objectives of the survey. They may be several hundred feet apart, or as close as 15 feet.
In marine seismic surveys the survey design is a little different, and instead of a static set of lines, a vessel tows behind it a series of streamers, each having a series of hydrophones along its length. See e.g., FIG. 1A and FIG. 1B. Also towed behind the vessel are one or more seismic sources.
A variety of seismic sources are available for marine applications, including water guns (20-150 Hz), Air Gun (10-150 Hz), Sparkers (50-4000 Hz), Boomers (30-300 Hz), and Chirp Systems (500 Hz-12 kHz, 2-7 kHz, 4-24 kHz, 3.5 kHz, and 200 kHz), but air guns are by far the most common.
The streamers also have depth control “birds” programmed to pivot their wings in response to hydrostatic pressure, thus keeping the streamers at a constant depth, as well as “paravanes” to minimize lateral deviation, described in more detail below. One of the most critical elements of 3D marine seismic systems is positioning. Thus, the vessel also tows one or more tail-buoys that house a differential global positioning receiver used to accurately position each of the hydrophones and additional navigation pods (GPS units and transceivers) are located on the paravanes, gun arrays and pretty much any other location that one can mount them above the surface of the water. Also, noise attenuation algorithms are now available (see e.g., Q-marine single sensor technology) that allow the collection of useful data, even when sailing in curves.
A seismic vessel with 2 sources and towing a single streamer is known as a Narrow-Azimuth Towed Streamer (aka “NAZ” or “NATS”). By the early 2000s, it was accepted that this type of acquisition was useful for initial exploration, but inadequate for development and production, in which wells had to be accurately positioned. This led to the development of the Multi-Azimuth Towed Streamer or “MAZ,” which tried to break the limitations of the linear acquisition pattern of a NATS survey by acquiring a combination of NATS surveys at different azimuths (see FIGS. 2A-2B). This successfully delivered increased illumination of the subsurface and a better signal to noise ratio.
The seismic properties of salt poses an additional problem for marine seismic surveys, as it attenuates seismic waves and its structure contains overhangs that are difficult to image. This led to another variation on the NATS survey type, the wide-azimuth towed streamer (aka “WAZ” or “WATS”), which was first tested on the Mad Dog field in 2004. See FIG. 3. This type of survey involved a single vessel towing a set of 8 streamers and two additional vessels towing seismic sources that were located at the start and end of the last receiver line (see diagram). This configuration was “tiled” 4 times, with the receiver vessel moving further away from the source vessels each time and eventually creating the effect of a survey with 4 times the number of streamers. The end result was a seismic dataset with a larger range of wider azimuths, delivering a breakthrough in seismic imaging.
Another common acquisition pattern for 3D seismic marine surveys is the “racetrack” vessel pattern, wherein the survey has a single line orientation (or “survey azimuth”), and a long, narrow spread of streamers are towed by a single vessel. Typically, a vessel equipped with one or two airgun sources and towing 8-10 streamers travels in a straight line through the survey area. When it reaches the edges of the survey area, it continues in a straight line for one half the length of a streamer then turns in a wide arc to travel in a straight line back and parallel to the first run. With each subsequent run, the racetrack like course is displaced laterally from the last run, until the entire area has been covered.
The racetrack pattern is shown FIG. 4, wherein the acquisition path follows a straight line (blue arrow) then turns 180° to acquire data in the opposite direction (orange arrow). No data are normally recorded during line turns (black) because the streamers do not maintain their lateral separation during turns and the position of the receivers cannot be accurately calculated. Further, there is known to be increased noise during turns due to dragging the streamer through the water somewhat sideways.
Recently, surveyors have developed a coil pattern, involving circles that gradually shift in the desired direction—a development made possible with Q-marine single sensor technology. See Biva (208). Compared to prior acquisition patterns, the coil pattern delivered a higher number of contributions (yellow and red) for a complete range of azimuths for all offsets. See FIGS. 5A-5E: narrow-azimuth (FIG. 5A), multiazimuth (FIG. 5B), wide-azimuth (FIG. 5C), rich-azimuth (FIG. 5D), coil shooting (FIG. 5E). Further, with parallel geometries, vessels are productive about 45% of the time, but with a coiled geometry, they are productive about 90% of the acquisition time.
During pre-processing, positional data gathered in the field is used to compute a theoretical grid network called a binning grid. Every individual recorded seismic trace is assigned to one or more bins; the number of traces summed together at each bin is called the fold or coverage for that bin. The nominal average fold for the survey is part of the descriptive information for the survey. Summing all the traces assigned to each bin creates a single multi-fold trace that is used as input to subsequent seismic processing steps. The general rule of thumb is that 3-4 bins are required to map the smallest (narrowest) horizontal dimension of a stratigraphic feature that must be seen in the 3D data volume. See e.g., FIG. 6. Thus, the geophones in a land-based survey are set at the optimal spacing to allow for 3-4 bin coverage of the smallest feature to be mapped.
However, in marine surveys, the normal approach is to select the bin grid size based upon the spacing of the sensors in the cables and the spacing of the streamers in the water. Since sensor spacing is fixed at the time of manufacture, most (if not all) marine seismic surveys are acquired at some multiple 12.5 meters (m). Thus, a common bin size is 6.25 m by 25 m or 12.5 m by 12.5 m. If geophysically one only needed a 16 m bin grid, the conventional approach would be to oversample at 12.5 m. However, this is expensive and wasteful, since these surveys can take months to perform. This conventional “racetrack” pattern generates a very uniform distribution of data over the project, but it is wasteful from a compressed seismic imaging approach as the survey acquires an excess of data that is unneeded.
Thus, what is needed in the art is a better method of establishing the bin grid pattern in marine 3D seismic surveys that optimizes data acquisition over the survey area, and doesn't needlessly cover or over-cover the geological features to be mapped.